Many resistivity sensors used in formation evaluation (FE) logging processes utilize a focusing technique, in which a guard electrode emits current in order to lead the current beam of a measurement electrode deeper into a conductive material. For these focused sensors, the driving potential on the guard and the measurement electrode must be exactly the same to avoid disturbances of the ideal electrical field, which makes sure that the focusing effect takes place. Driving potential differences may lead to currents flowing between the guard and the measurement electrode, which would completely destroy the focusing effect and lead to high measurement errors if not considered. An example of a focused sensor is described in U.S. Pat. No. 6,025,722 entitled “Azimuthally segmented resistivity measuring apparatus and method”.
Other focused resistivity devices utilize a shielding electrode to improve focusing. An example of a resistivity device utilizing a shielding electrode is described in U.S. Pat. No. 7,256,582 entitled “Method and apparatus for improved current focusing in galvanic resistivity measurement tools for wireline and measurement-while-drilling applications”. Such devices have limitations in the ability to focus, thus leading to limited penetration depths. Furthermore, typical devices utilize a single voltage source that supplies the measurement and shielding electrode, which may also limit the focusing ability of the device and/or limit the ability to precisely control the driving potential applied to the measurement electrode and the shielding electrode.